This invention relates to planetary gear units, and more particularly to a planetary gear unit which may be employed, for instance, as the speed reducer of a starter.
A conventional planetary gear unit employed, for instance, in the speed reducer of a starter, as shown in FIG. 5, comprises: a sun gear 1 mounted on the shaft of a prime motor such as an electric motor (not shown); an internal gear 2 arranged coaxially with the sun gear 1; and three planet gears 3 engaged with both the sun gear 1 and the internal gear 2.
The internal gear 2 has teeth 2a formed in its inner cylindrical wall at equal intervals.
The three planet gears 3 are supported by a carrier (not shown) at equal angular intervals in the direction of revolution.
The carrier is coupled to the output shaft (not shown).
When employed as a speed reducer for a starter, the planetary gear unit thus constructed is engaged with the starter as follows: The internal gear 2 is engaged with the frame 5 of the starter in such a manner that protrusions 2b formed on the outer cylindrical wall of the internal gear 2 are engaged with recesses 6 formed in the inner cylindrical wall of the frame 5, so that the rotation of the internal gear 2 around its axis is prevented.
In FIG. 5, reference numeral 7 designates through-bolts of the starter (such as a coaxial starter).
The starter employing the above-described conventional planetary gear unit suffers from the following difficulties: When something exists outside the frame 5 obstructing it partially, it is necessary to miniaturize the planetary gear unit to reduce the outside diameter of the frame 5.
However, the miniaturization of the planetary gear unit give rise to another problem.
That is, the planetary gear unit cannot be miniaturized without reduction of the gear ratio or gear module.
However, the reduction of the gear ratio or gear module makes it difficult to obtain the predetermined torque and speed, or to maintain the predetermined characteristics and gear strength.
This problem occurs not only with the starter but also with other devices incorporating the planetary gear unit.